This invention relates to a control algorithm for moving from the end of a defrost transient back to steady state operation in a refrigerant cycle.
Refrigerant cycles are utilized to provide cooling, such as for refrigerant systems, or air conditioning systems. In a typical refrigerant cycle, a compressor compresses a refrigerant and delivers a hot refrigerant to a downstream condenser, which is typically located outside. From the condenser, the refrigerant passes to an expansion device at which the refrigerant is expanded and cooled. The cool refrigerant then passes into an indoor heat exchanger known as an evaporator. A fan typically passes air over the evaporator, and the air is cooled by the refrigerant. This cooled air is then passed into an environment to be conditioned.
In a standard air conditioning application, this air is brought to a temperature that is comfortable for human occupation. In refrigeration applications, the air is brought to much cooler temperatures.
One periodic maintenance function that must be performed on such a refrigerant cycle is a defrost cycle. Ice will cover the coils of the evaporator, and periodic defrosting is necessary to melt this ice. The defrost cycle may be performed by turning on a heater coil that is positioned adjacent the evaporator. Typically, when the defrost cycle has been initiated, the compressor is stopped. Further, the fan at the evaporator is stopped.
Once the defrost cycle has ended, and the heater has melted the ice on the evaporator, there is liquid water on the evaporator coils. For this reason, the fan is not typically started immediately when the compressor is again started. Especially in refrigeration applications, the fan would carry the liquid water with the air and could do some damage to goods within the space to be conditioned. Thus, in at least some refrigerant cycles, the compressor is started at the end of defrost cycle but the evaporator fan stays off for an additional period of time. At some point soon after initiation, the refrigerant in the evaporator freezes the remaining moisture. At this point, the fan is started.
There are problems with the above prior art method of operation. In particular, the expansion device that is upstream of the evaporator is typically controlled to meter the amount of refrigerant being delivered to the evaporator dependent on the temperature at the evaporator. During the period soon after the end of the defrost cycle, even in refrigerant cycles where the fan begins to operate immediately, the evaporator coils are relatively hot. At the end of the defrost cycle, the metal evaporator coils have typically been heated by the heater coils.
A control for the expansion device would interpret this hot evaporator as being indicative of a need for increased refrigerant flow. Thus, there is a concern with two much refrigerant being delivered through the expansion device into the evaporator at the end of a defrost cycle. This increased refrigerant flow could lead to flooding in the evaporator and compressor, which is undesirable.